Leachate: production and characterizaton

Leachate production deals with the
creation of contaminated liquid at the base of a landfill. It involves the elements of a
water balance in which precipitation either runs off from the landfill or infiltrates.
Some infiltration will evapotranspire, some may be stored within the landfill, and the
balance becomes percolate and eventually leachate. Examples of the water balance method
are presented and consideration is given to the sequence of landfill development and the
effect that it has on leachate flow.

Information is provided on the microbial
decomposition of municipal solid waste and the conditions which influence decomposition
rates. The impact of the microbial processes on leachate contaminant composition is
examined. Leachate characterization requires that estimates be made of contaminant types
and concentrations as a function of refuse age. Data describing contaminant production
from various sources are examined. A procedure is presented to combine the water balance
method and contaminant production curves to predict leachate flow and strength with
respect to site age. A simpler alternative involving a set of tables showing expected
contaminant types and ranges of concentrations as a function of refuse age is also
provided.

Introduction

Landfills have served for many decades as
ultimate disposal sites for all manner of wastes: residential, commercial, and industrial,
both innocuous and hazardous. Landfill technology has evolved from the open, burning dump
to highly engineered sites designed to minimize tile impact of contaminants in the waste
on the adjacent environment.
The major environmental problems experienced at landfills have resulted from the loss of
leachate from the site and the subsequent contamination of surrounding land and water.
Improvements in landfill engineering have been aimed primarily at reducing leachate
production, collecting and treating leachate prior to discharge, and limiting leachate
discharge to the assimilative capacity of the surrounding soil. Whether leachate is to be
collected and treated or is allowed to discharge to the soil, it is essential to have
estimates of leachate flow and strength and the variation of these with time as the site
develops, through closure and after closure. While these estimates are essential to proper
landfill design, their preparation is a difficult and uncertain process. This paper
examines current methods and data available for estimating leachate production and
variability.
________________________
Note: Written discussion of this paper is welcomed and will be received by the Editor
until October 31, 1989 (address inside front cover).

Mechanisms of leachate formation

Leachate is produced when moisture enters
the refuse in a landfill, extracts contaminants into the liquid phase, and produces a
moisture content sufficiently high to initiate liquid flow. Sources of moisture entering
the landfill include liquid present in the refuse at placement, precipitation falling on
refuse at placement and infiltrating after cover application, and intrusion of groundwater
from outside into the landfill.

A generalized pattern of leachate
formation is presented in Fig. 1. The components shown include the following steps:

Precipitation (P) falls on the landfill
and some of it becomes runoff (RO).

Some of P infiltrates (I) the surface
(uncovered refuse, intermediate cover, or final cover).

Some of I evaporates (E) from the
surface and (or) transpires (T) through the vegetative cover if it exists.

Some of I may make up a deficiency in
soil moisture storage (S) (the difference between field capacity (FC) and the existing
moisture content (MC)).

The remainder of I, after E, T, and S
have been satisfied, moves downward forming percolate (PERC) arid eventually leachate (L)
as it reaches the base of the landfill.

PERC may be augmented by infiltration of
groundwater (G). The procedure used to analyze these processes is referred to as a water
balance (WB), various forms of which are commonly

used for the simulation of surface water
hydrology. The algebraic statement of this form of water balance is

[1] PERC = P - RO - ET - AS + G

While [1] is conceptually correct and
comprehensive, accurate predictions of leachate flow are difficult to achieve because of
the uncertainties associated with estimating the various terms. Most formulae and methods
in use are empirical. Some of the data base required is stochastic in nature (temperature,
heat index, precipitation, wind, vegetative growth). Other data are poorly defined (runoff
coefficients, refuse arid cover density and compaction, moisture storage capacities).

Analyses have been performed to compare
water balance predictions of leachate flow with actual measurements made in the field. Gee
(1981) used two variations of the water balance method to predict leachate flow at the
GROWS Landfill in Bucks Co., Pennsylvania. These predictions were too high by a factor of
approximately 2 when compared with measured leachate flows. Lu et al. (1981)
performed similar comparisons at 5 landfills using 25 different methods to estimate the
various terms of [1]. On average, leachate flow estimates were in error by a factor of 2.
However, the poorest estimates were as much as 100 times greater than the measured
leachate flows.

In contrast, Kmet (1982) had excellent
success using a water balance method to simulate leachate production in Ham's (1980) eight
field lysimeters. Leachate flows ranged from 16.6 to 22.1% of precipitation on an annual
average basis. Water balance methods predicted an average of 22% of precipitation,
providing excellent agreement with measured values. Kmet used the water balance method
(WBM) proposed by Fenn et al. (1975)with modifications to account for
infiltration and runoff from the landfill during winter conditions. This appears to be an
acceptable procedure to predict leachate flow.

The WBM as proposed by Fenn et al. (1975)is a manual procedure solved generally with monthly averaged values. Computer models
have been developed subsequently using the WBM as a basis with various modifications. The
hydrologic simulation of solid waste disposal sites (HSSWDS) model developed by Perrier
and Gibson (1981) and the hydrologic evaluation of landfill performance (HELP) model
reported by Schroeder (1983) are two of the more widely accepted of these computer models.
The HELP model is perhaps the best of the available computer models. Its use has become
compulsory for Superfund Site evaluation.

Components of the WBM and the
HELP model
The component steps of the WBM used to calculate landfill leachate flow are presented in
flow chart form in Fig. 2. Table 1 provides a summary description of each step and
compares the steps with

those of the HELP model. More detailed
information about the methods is given in Kmet (1982, 1986). Tile components are generally
calculated in units of height of water per unit time (e.g., centimeters (or inches) per
month).

The time of leachate arrival at the base
of the landfill is handled differently in the two methods. The WBM does not account for
the period of time required for the refuse to be brought up from its moisture content (MC)
at placement (e.g., 15 cm.m-1) to field capacity (FC) (e.g., 35 cm.m-1)
at which point liquid flow begins. This can take several months depending on the refuse
type, compaction, and depth in addition to the percolation rate. The WBM assumes that the
refuse is already at FC and that a unit of PERC at the top produces an equivalent unit of
leachate at the bottom. The method is therefore applicable only after FC has been reached
in the landfill. It is possible to estimate the time of first leachate production using
PERC and allowing it to increase the MC until FC is reached. However, Kmet (1986) has
shown that the actual time at which leachate first appears at a landfill is much less than
that predicted with this method. He attributed this to channelling and nonhomogeneous MCs
and flow properties within the refuse. This would result in FC in the region of channels
much less than FC at other locations within the refuse.

The HELP model, in addition to performing
a water balance, also calculates a flow rate through the refuse and therefore estimates
the time of first leachate appearance. However, channelling due to heterogeneities within
the refuse reduces the accuracy of these flow rate calculations and tends to overestimate
leachate arrival times.

FARQUHAR

Table 1. Components of the WBM and
comparison with the HELP model
============================================================
WBM
HELP model
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Output of theWBM
The output from the WBM and also the HELP model (although the Iatter is more
comprehensive) is represented in Fig. 3. The data are given as monthly averages and thus
some seasonality is evident. Leachate production (PERC) exists in the months from February
to May inclusive and is zero during the remaining months. Seasonality in the other
components such as P and ET is also shown. When PET exceeds I, S is reduced by
evapotranspiration as shown in the months of June, July, and August. In November and
December, although I exceeds PET no PERC occurs since l is used to make up the deficit in
S (SMAX = 9 cm in this illustration).
The output therefore provides estimates of leachate flow and its variability throughout
the year. Such information is essential when assessing the impact of the leachate on
either the soil environment or a treatment facility to which the leachate is being
discharged.

Predicting leachate flow under changing
landfill conditions

An assumption inherent in the previous
section is that the landfill conditions remain constant during the analysis. While this is
essential to the analysis, it must be acknowledged that the landfill conditions are not
uniform throughout the landfill at any time and that these conditions will change as the
site ages. Some sections of the landfill may have final cover while others may have
intermediate cover or no cover at all as at the working face. Both the area and the depth
of the landfill will increase with time. As well, at any time, the refuse in the landfill
ranges in age from new to old and will therefore have been exposed to different amounts of
PERC. Field-scale calculations of leachate flow must take these variations into account.

Figure 4 shows a schematic representation
of landfill development and leachate flow rate change as the landfill grows in size and
depth with time. The leachate flow rate increases as the surface area of the landfill
increases. The flow rate decreases after the application of less permeable soils and
vegetation for the final cover.

The impact of leachate on the environment
or on a system for collection, treatment and disposal will be influenced by these
variations in leachate flow. Consequently, the WBM or HELP model simulations must be done
sequentially to account for the changing conditions throughout the life of the landfill.

The leaching process

As PERC moves through the refuse,
contaminants are mobilized into the liquid phase through dissolution and suspension from
the stationary refuse phase, thus producing a contaminated leachate. Increased moisture
enhances microbial activity within the landfill. As a result, metabolic by-products such
as volatile fatty acids and alcohols are contributed to the leachate, increasing its
organic strength. Some organic compounds augment the leaching potential of the liquid
because of increased acidity and complexing potential.

Table 2 provides information on the
composition of municipal solid waste based on several analyses performed throughout Canada
and the United States. The component compositions are presented as ranges of percent wet
weight of refuse. Category A consists of readily biodegradable food and garden wastes
which produce high concentrations of organic matter (as BOD or TOC) and total Kjeldahl
nitrogen in the leachate. This often occurs within the first few months of leaching.
Category B is also organic but less biodegradable than A. It includes primarily newsprint
and other paper with much smaller amounts of wood and rubber as examples. Because of
reduced biodegradability, these components yield organics to the leachate at
concentrations much lower than for Category A but for much longer times measured in years.

Category C includes metallic wastes
composed mainly of iron, aluminum, and zinc. In time, these and other metals appear in the
leachate and do so for many years because of slow rates of release.

Category D includes nonmetallic inorganic
components such as glass, soil, and salts. The readily soluble of these appear in the
leachate in the first few months of leaching, while the less soluble will yield
contaminants for several years. The alkaline earth metals (calcium, magnesium, sodium, and
potassium) and the common anions (chloride, sulphate, phosphate, and carbonate) arise
mainly from these waste components. In all cases, there is a limit to the amount of
contaminant that can be leached from the refuse.

Because of these trends and conditions,
the concentration of most contaminants in the leachate varies with time. Most
contaminants, especially biodegradable organics, tend to reach peak concentrations in the
leachate in the earlier months of leaching and then reduce subsequently. However, some
contaminants such as poorly biodegradable organics and iron tend to persist in the
leachate for several years. This is shown in a generalized way in Fig. 5. Information
about typical leachate contaminant types and concentration ranges as they change with time
is presented subsequently.
Figure 4 shows a schematic representation of landfill development with time in which each
year's refuse deposition is identified by a number. At any time, for example during the
10th year, some refuse is fresh and not leached at all, while other refuse has been in
place and exposed to leaching for 10 years. Each year's refuse will have a different age
and thus will be at a different point on the time axis in Fig. 5. The older refuse will be
producing leachate with contaminant concentrations represented by the right-hand side of
Fig. 5, while the left-hand side applies to younger leachates. Thus the leachate produced
in the 10th year will have contaminant concentrations which are weighted averages. They
will be averaged from different sections of the landfill having refuse of different ages
and different leaching histories. It is also apparent from this analysis that contaminants
are contributed to the leachate for many years after the site is closed.

Microbial degradation of refuse in
landfills

Except for the first few days after refuse
placement, microbial decomposition in landfills proceeds under anoxic conditions.
Hydrolytic and fermentative microbial processes solubilize waste components, producing
organic acids, alcohols, ammonia, and carbon dioxide as major products. These processes
are vigorous and rapidly initiated as the moisture content increases in the landfill.
After several months, methanogenesis is initiated with methane and carbon dioxide produced
as by-products. Methanogenesis is a slower and more fastidious process.
Microbial degradation in landfills has been described in detail by Farquhar and Rovers
(1973), Fungaroli and Steiner (1979), Zehnder (1978), and Ham (1980) and will not be
examined in detail here. Two concepts are important to establish, however:

(1) active methanogenesis
substantially reduces leachate organic strength (by decomposing organic acids and
alcohols) and increases pH; and (2) vigorous methanogenesis does not always occur in
landfills because the landfill environment is much less than optimum for the methane
bacteria (see Table 3).

Predicting leachate ocntaminant
concentrations

The types, amounts, and production rates
of contaminants appearing in the leachate at a landfill site are influenced by several
factors: refuse type and composition; refuse density, pretreatment, placement sequence,
and depth; moisture loading to refuse as influenced by the factors described for the WBM;
temperature; and time.

Accurate quantification of these factors
and their impact in the very heterogeneous conditions found in a landfill is difficult.
The mechanisms and extent to which they influence contaminant release are poorly
documented. It is therefore necessary to rely on data and experience from other landfill
investigations and to apply them to landfills under study. Caution is necessary, since the
conditions of the landfill investigation are often not documented and may not be the same
as those being considered.

Several studies have been undertaken since
the late 1960's in which investigators have collected leachates from refuse under various
conditions and have measured their contaminant concentrations. Some have involved actual
landfill sites, but the majority have been conducted with the use of lysimeters to
simulate landfills. The lysimeters have ranged from small (volume <1 m3)
laboratory units to large (volume >10 m3) field units. Lysimeters have often
been preferred by investigators because it is easier to control conditions and make
measurements of the leaching process with a lysimeter than with a full-scale landfill.
Unfortunately, many lysimeter studies have not successfully simulated landfill conditions
and have therefore produced atypical leachate. In comparison with the representative
southern Ontario landfill conditions given in Table 3, lysimeters have often been too warm
(+2O°C), too wet (several times greater than natural PERC), too shallow (<1 m), and
too short in duration.

Lysimeter leachates have tended to be
stronger than those experienced in the field, often because methanogenesis was not
vigorous.

Leachate contaminant production curvesLu et al. (1985) have produced an extensive review of investigations reporting
leachate production and contaminant concentrations. They have combined the data obtained
from these studies to produce contaminant production curves similar to those shown in Fig.
5. Plots for BOD5, iron (Fe), chloride (Cl), and ammonia nitrogen (NH3-N)
have been reproduced in

Fig. 6. As might be expected, the data
exhibit substantial scatter due to the wide range of conditions under which the studies
were performed. In particular, some studies made use of atypical lysimeters and generally
produced the higher concentrations shown. Consequently, the plots and models produced by
Lu et al. (1985) represent upper limits for leachate contaminant concentrations at field
installations.

Some investigators (Fungaroli and Steiner
(1979), Ham (1980), Wigh and Brunner (1981), and McGinley and Kmet (1984)) have produced
data which appear to reflect field conditions more closely. Their work has also been
conducted and reported in such a way that the impact of some of the important factors such
as compacted density, moisture addition, depth, and refuse age can be evaluated. These
data sources are therefore particularly important in the prediction of leachate
contaminant concentrations.

McGinley and Kmet (1984) have attempted to
combine the data from these more realistic studies to produce leachate contaminant
production curves. Examples of their plots of several data sets are shown in Fig. 7. One
data set has been eliminated from each of the upper and lower graphs in Fig. 7. (These
appeared to be clearly different from the others.) The graphs show leachate chemical
oxygen demand (COD) as a function of moisture loading to the refuse in units of litters
(L) of leachate per kilogram (kg) dry refuse (as opposed to time) in an attempt to
normalize the data. The upper graph expresses COD as mg.m.L-1 in the leachate
while the Iower one uses mg COD leached per kg dry refuse. While the data are scattered
there is a reasonably good trend shown in each case. Young leachates exhibit CODs in the
range of 30 000 to 50 000mg.L-1, while leachates from old, extensively
leached refuse have CODs generally less than 2000 mg. L-1.

Similar plots for other leachate
contaminants were prepared by McGinley and Kmet (1984). Most data tend to level off at
moisture Ioadings of 5 L.kg-1, indicating that the limit of leachate
contaminants has been reached.

Method for predicting teachate contaminant
concentration
Figure 8 (adapted from the chloride (Cl) graphs of McGinley and Kmet) presents information
for developing a rational method to calculate leachate contaminant concentrations in a
field situation. It shows four cells of a landfill, A to D, one above another. The
geometry and the dry refuse mass density are the same for all cells in this simplified
illustration. However, each cell has been placed at a different time and thus has a
different age and leaching history; the latter is given as:

PARQUHAR

Table 4. Example to calculate
leachate chloride concentration

A WBM analysis has been performed and has
predicted that the leachate percolation at this time is 0.1 m/month or 0.6 m for the
6-month time interval used in this analysis. This liquid will flow through all four cells
in sequence, picking up contaminants as it goes. Thus each cell will move along the
contaminant production curve by an amount equal to

during the 6-month study interval as
shown. Table 4 summarizes the steps required to calculate the leachate chloride
concentration as an average over the 6-month interval and for the 0.6 m of PERC produced.
The leachate chloride concentration is calculated to be 4000 mg. L-1 on average
as it discharges from the lowest cell D.
Certain trends can also be established from this example:
1. It can be shown that the leachate chloride concentration will be lower in the next
6-month period since each cell has been moved to the right on the contaminant production
curve.
2. Other "stacks" of landfill cells adjacent to the one shown must be analyzed
in the same way. The total leachate chloride concentration will be the volume weighted
average from all stacks.
3. The analysis must be repeated for all contaminants.
While this method of calculating leachate contaminant concentrations is rational, it has
weaknesses which impede its use at the present time:
1. Production curves such as those shown in Fig. 8 for chloride have not yet been prepared
for all important landfill leachate contaminants.
2. This "rational method" has not yet been validated against field data.
3. The method is cumbersome and must be computerized and integrated with the leachate flow
calculations produced by the WBM or HELP model.

Typical landfill leachate contaminant
types and concentrations

The tendency in the past, in the absence
of a functional rational method based on leachate flow and contaminant production curves,
has been to rely on experience for estimating landfill leachate contaminant
concentrations. Experience has been based on reported leachate contaminant concentrations
from various sources. Unfortunately, the information has not been formalized into unified
categories with respect to important influential factors such as depth, moisture loading,
and age, for example. In this absence, Tables 5-8 have been prepared in the format of
leachate contaminant ranges as a function of site age. The background for the tables comes
largely from the work of McGinley and Kmet (1984) with additional input from publications
by Fungaroli and Steiner (1979), Ham (1980), and Lu et al. (1985).

The tables are presented for different
classes of contaminants with expected concentration ranges reported for four refuse age
categories, 0-5, 5-10, 10-20, and >20 years old. Age categories have not been provided
for trace metals and trace organic priority pollutants because of a lack of data
available. It is acknowledged that leachate contaminant concentrations at certain sites
may differ from the information provided in the tables owing to specific conditions at
those sites.

Table 6. Concentration changes with
refuse age - major cations and anions

The tables should be used in conjunction
with the WBM, the HELP model, or some equivalent method to produce leachate flows and
contaminant concentration patterns over the active life of the landfill. The information
will be approximate, but such estimates are essential for assessing the impact of landfill
leachates on surrounding soil/groundwater environments and on wastewater treatment
facilities.

It is anticipated that, in the near
future, procedures will be available, integrating the WBM and HELP model with extended
contaminant production curves, to provide more accurate predictions of landfill leachate
generation.

Summary

It is essential that the engineers who
design and operate sanitary landfills be able to predict leachate flow and composition for
as long as the site remains active. This is needed either for the purpose of leachate
treatment or for the discharge of leachate to the environment. The HELP model, in which
the central function is to perform a water balance, is considered by many to be the best
means currently available to predict leachate flows. This is notwithstanding the
experience of some designers who have found that significant differences exist between
actual leachate flows and those generated by the HELP model.

The methods to predict leachate
composition are much less formalized than those for leachate flow. The data are poorly
organized and not easily available to the designer for use in estimating leachate
strength. As a result, this research was undertaken to provide assistance in this regard.
Tables have been created to present typical contaminant concentrations as a function of
site age. The trends and data were based on experience and an extensive review of
pertinent technical literature. While these tables improve the ability to predict leachate
strength, they do not take into account the variations which exist from site to site, the
most important of which are moisture loading and site geometry. In response to these
shortcomings, the paper also presents a novel approach to predicting leachate composition.
It uses data on site geometry and moisture loading as produced from some form of water
balance such as the HELP model and combines it with contaminant leaching curves to produce
leachate concentrations. An example of the method applied to chloride ion is provided;
however, much work remains to be done on the method before it can be made available for
general use.